Covid-19 was circulating in Ireland at least two weeks before the first positive case was identified, a new study suggests.
Samples taken from two blood donors reveal they had Covid-19 antibodies on February 17th of last year, 12 days before the first Irish case of the disease was detected by health authorities. The donors were identified separately in Munster and in Ulster and both were asymptomatic.
The study, published in the Journal of Clinical Virology, analysed 8,509 blood samples taken between February and September of last year. Covid-19 was found in 187 (2.2 per cent) of the samples tested.
When adjusted for the overall population, as people under the age of 18 do not give blood, the rate was 2.4 per cent. This is a much higher rate than that which prevailed in society at the start of October 2020, by which time 0.75 per cent of the population had tested positive for Covid-19.
It suggests that in the first wave the number of Covid-19 cases in the community was at least three times the number detected through PCR tests.
Prof Allison Waters, a researcher at the Irish Blood Transfusion Service (IBTS), said the results were consistent with studies elsewhere which showed higher rates of Covid-19 in the community than that detected through testing.
‘Don’t know it’
“Long before we had Covid, we had flu every year and the only people getting tested are those with symptoms. It does not mean it is not circulating constantly in the community,” she said. “It is certainly likely that many of us have had Covid-19 and don’t know it.”
Prof Waters said the trajectory for Covid-19 suggests that the disease will eventually have circulated so widely that it becomes epidemic in society.
“Other infectious respiratory diseases show that throughout our lifetime, we build up some pre-existing immunity. You expect that the virus will keep changing, but those changes are such that a little bit of your immune response will still be able to react to them,” she added.
The study shows that the spread of Covid-19 began slowly in February and March of last year. It rapidly accelerated in April, during the peak of the first wave, to 2.91 per cent of the samples tested from that month.
The seroprevalence rate stabilised at between 2.5 and 3.5 per cent for the remainder of the study period. The reported showed that Dublin was significantly more affected by Covid-19 than the rest of the country during the first and second waves.
